1. Field of the Invention
The present invention relates to uretdione-group-containing polyaddition compounds, to a process for the their preparation and to their use in the manufacture of polyurethane plastics, especially cross-linking agents for thermally cross-linkable powder coatings.
2. Description of the Prior Art
Compositions of organic polyhydroxy compounds and blocked polyisocyanates, which are solid at room temperature, are valuable binding agents for thermally cross-linkable powder coatings (see, for example, DE-OS 2,105,777, DE-OS 2,542,191, DE-OS 3,143,060, DE-OS 2,735,497, DE-OS 2,842,641, EP-A 286,799, EP-A 218,040, EP-A 460,963, DE-OS 2,801,126, EP-A 403,779, WO 91/15532, U.S. Pat. Nos. 3,857,818, 4,375,539, EP-A 409,745, DE-OS 2,812,252, DE-OS 2,946,085 and DE-OS 3,434,881).
Common to the systems of those prior publications is the disadvantage that, during thermal cross-linking, the compounds used as blocking agents are split off and escape into the environment. Therefore, when they are being processed special precautions must be taken to purify the waste air and/or to recover the blocking agent for reasons of ecology and work hygiene.
An attempt to eliminate that fundamental disadvantage can be seen in the use of blocking-agent-free, uretdione-group-containing polyurethane (PUR) powder coating hardeners, in which cross-linking takes place with thermal re-cleaving of the uretdione groups.
The preparation of such powder coating cross-linking agents is known in principle. For example, DE-OS 2,420,475 describes reaction products of uretdione-group-containing polyisocyanates or polyisocyanate mixtures and difunctional and, optionally, monofunctional aliphatic or cycloaliphatic compounds, carrying groups that are reactive towards isocyanates, as blocking-agent-free cross-linking agents for PUR stoving systems. Solid products that are suitable as powder coating hardeners and have melting points in the range of from 140.degree. to 260.degree. C. are formed especially when uretdione-group-containing polyisocyanates based on aromatic diisocyanates, such as toluylene diisocyanate (TDI), are reacted with simple diols optionally containing ether groups.
Since cleaving of the uretdione ring in the presence of hydroxy-functional reactants is initiated at temperatures as low as approximately 110.degree. C., the described polyaddition products suitable as powder coating cross-linking agents cannot be prepared in the melt but only dissolved in solvents that are inert towards isocyanates, which then necessitates an additional process step in order to separate the auxiliary solvent.
Uretdione group-containing powder coating cross-linking agents prepared from linear, difunctional IPDI uretdione, simple diols optionally containing ether oxygen, and, optionally, terminal monoamines or monoalcohols are the subject of EP-A-0,045,994, EP-A-O,045,996 and EP-A0,045,998. In the preparation of these polyaddition products the reaction temperature also must not exceed 110.degree. C. because otherwise cleaving of the uretdione groups is initiated. When a high degree of chain lengthening is necessary to obtain a high uretdione functionality of the cross-linking agent and also a high cross-linking density (i.e., when at least 70% of the NCO groups of the difunctional IPDI uretdiones are reacted with diols) the resulting chain-lengthened products have melting points in the range of the cleaving temperature or higher. Therefore, such polyaddition compounds can only be prepared in a reliable and reproducible manner in solution with a subsequent evaporation step.
The IPDI uretdione powder coating hardeners, described in the preceding publications, have melting points in the region of approximately 80.degree. C., which are clearly below the cleaving temperature. However, these products still have such high melt viscosities at the maximum permissible reaction temperature of 110.degree. C. that their ability to be produced in the melt on an industrial scale is limited due to the fact that they cannot be stiffed to a sufficiently high degree.
In addition, powder coatings prepared using the known uretdione group-containing powder coating hardeners of the prior art generally exhibit only moderate optical properties, especially insufficient flow, due to the high melt viscosity of such compounds.
An object of the present invention is to provide uretdione group-containing PUR powder coating cross-linking agents that do not have the described disadvantages of the polyaddition products of the prior art.
This object may be achieved with the polyaddition compounds according to the invention described in more detail hereinafter and the process for their preparation. These polyaddition compounds are based on the surprising observation that uretdione group-containing polyaddition compounds containing ester and/or carbonate groups have such low melt viscosities, even at temperatures only slightly above their melting point, that they can be prepared without difficulty in the melt and can be used to obtain powder coatings that are distinguished by excellent flow and a very high degree of gloss. An especially surprising fact is that when these uretdione group-containing polyaddition compounds are used as hardener component in PUR powder coatings, they harden to form fully cross-linked coatings at much lower stoving temperatures than do the corresponding polyaddition compounds of the prior art which do not contain ester and carbonate groups.